Generate audit event when merge request rule is automatically approved and marked as Invalid

Problem to Solve

In #334698 (closed) messaging was updated to better convey when an approval rule was automatically approve because the configuration was invalid for some reason (most common: one approver who is also the author). This change helps to highlight within the merge request when this situation occurs, but there is not any record of the situation occurring.

Proposal

A new event should be added to the audit events that logs an event whenever a merge request approval rule is marked as Invalid in the UI.

Additional Details

This needs to be added as an Audit Event initially for internal needs, but likely also needs to be added to the streaming audit events in a follow-up.